Many of today's electronic systems require ultra-small size to meet consumers expected hearable and wearable functions with long on battery life despite their tiny form factor. However, the device size does limit the battery capacity. People expects their hearables, wearables, and other tiny, battery-powered electronic devices to perform reliably over long period of time.
To support the operation of these tiny, battery-powered electronic devices, from design point of view, the form factor limitations dictate the need for a small Li+ battery, which must last for a long time between charge cycles. Power supplies must meet the distinct and diverse voltage requirements of each sub-system within the design. Engineers face increasing challenges trying to pack all the necessary product features into a tiny form factor of an earbud or a wearable gadget such as ring, bracelet, or skin patch.
For meeting the requirement of small solution size for easy installation and low thermal fluctuation, tiny low-power power management ICs (PMICs) using space-saving single-inductor multiple-output (SIMO) technology are suitable for application of these systems.
A single-inductor multiple-output (SIMO) architecture provides a better solution for tiny devices requiring good thermal performance, by integrating functionality in smaller devices that would otherwise require multiple discrete components. The concept of SIMO DC-DC converters is arisen in order to overcome the disadvantage of conventional converter, such as complexity and high cost, especially, the need for multiple inductors and controllers. Since a SIMO DC-DC converter can support multiple outputs while using only one inductor, it is an excellent candidate to minimize the component count and thus reduce the production cost. Apparently, the area of print circuit board can be reduced greatly, thereby miniaturizing devices.
A related art example as depicted in FIG. 1a, a SIMO DC-DC converter can support four output stages (VO1, VO2, VO3 and VO4) while using only one inductor (L) which operates at time-multiplexing control scheme (TMC scheme) via a SIMO Control Circuit & Logic circuitry 101 to control the turning on/off periods of all switches and to generate duty cycles. The duty cycle signals must be generated to control the input switches SP, SN, the freewheel switch SF and output switches S1, S2, S3, and S4, respectively, for the voltage regulation of each output. For supplying each output node, the inductor is charged by a duty cycle to get a required energy for its corresponding output before discharged the required energy to the corresponding output load. Consequently, for a complete conversion cycle, as illustrated in FIG. 1b, the inductor is charged four times by connecting the inductor between Vin and ground, and is discharged to zero current four times by connecting the inductor between ground and each output. The energy delivered for each output is well controlled and independent of other outputs so that cross regulation is removed. However, the peak inductor current of the TMC scheme is large since the inductor current is charged from zero and discharged to zero for each output. The inductor current is operating at discontinuous conduction mode (DCM). Therefore, the total output current capability is limited and many charging/discharging cycles with high peak inductor current cause high switching loss and low conversion efficiency. Moreover, the TMC scheme also suffers from the trade-off between the output voltage ripple and the number of outputs. Increase of the number of outputs requires longer time to regulate and thus results in higher voltage ripple.
In order to improve the power delivery capability, conversion efficiency and voltage ripple in TMC scheme, ordered-power-distributive control (OPDC) scheme distributes magnetic energy of the inductor to all the outputs sequentially at the same inductor energizing period. As shown by FIG. 1c, a charging duty cycle includes both charging the inductor current and discharging to output nodes, where each output switch is turned on at one time to share the inductor current. Since all outputs are regulated in one period, OPDC scheme can produce smaller voltage ripple for relative larger numbers of output channels. The inductor current is not necessary to be discharged to zero so that it can operates at continuous conduction mode (CCM), which has smaller peak inductor current. Therefore, smaller switching loss and higher output power delivery can be achieved.
However, due to the demand increased power efficiency in PMICs for hearables, wearables, and other tiny, battery-powered electronic devices, SIMO DC-DC converters as the key device should be also operated under various load conditions, such as continuous conduction mode (CCM) in heavy load condition, discontinuous conduction mode (DCM) in light load condition, and pulse skipping mode (PSM) in extreme light load or no load condition. Furthermore, the battery voltage is varying with the usage time. Under this application, auto-buck-boost function sometimes is required for the highest voltage channel. Therefore, a more advanced control scheme to optimize all operation modes and to have a buck-boost output channel is still demanding for realizing SIMO DC-DC converter into real applications.